The beta-thalassemias and sickle cell disease represent the most common congenital red cell disorders resulting from genetic defects in beta-globin gene expression. Recently, hydroxyurea (HU) has been shown to increase fetal hemoglobin (HbF) production in sickle cell patients and is also under evaluation as a treatment for beta-thalassemia. However, the effects of HU treatment on globin gene expression, particularly at the mRNA level, is not well understood. In order to improve our understanding of the mechanisms of action of HU, two HbSS patient and two beta-thalassemia patient (with IVS-2 nt 654 cytosine to thymine, CD 41-42 mutations) who received HU treatment were studied for changes in alpha, beta, and gamma mRNA levels by our newly developed competitive polymerase chain reaction (PCR) method. PCR quantitation was achieved by reverse transcriptase (RT) production of cDNA from mRNA in nucleated peripheral blood cells and subsequent competitive analysis of absolute mRNA levels. The results showed that the gamma-globin mRNA amount was increased by about 50-fold and the absolute magnitude of the change in gamma-mRNA levels were similar in the SS and beta-thalassemia patients. Increases, but to a smaller extent, were also observed in alpha and beta globin mRNA levels. These results suggest that the molecular mechanism of HU (and perhaps other related compounds) is a general one on expression of all globin genes, but greatest for gamma. Current studies in progress are aimed at: (1) investigation of the correlations between the globin mRNA levels and the reticulocyte numbers in nucleated cells when using the competitive RT-PCR method for globin mRNA assay; (2) investigation of globin gene expression (mRNA levels) by RT-PCR method in erythroid cultures treated with hydroxyurea; (3) investigation of globin gene expression (mRNA levels) in HU treated erythroid cultures from the patients with different types of beta-thalassemia mutations; (4) globin chain assay in HU treated patients as well as erythroid cultures by micro- globin biosynthesis assay. Thus, the overall objective is to contribute to the knowledge of the effects of HU on genetic regulation of globin production, and to provide us with understanding of the gamma to beta or beta to gamma switches in the synthesis of hemoglobin and how these switches may be modulated by drugs such as HU and erythropoietin.